A known fluid power assist steering system includes a pump for pumping hydraulic fluid to a fluid power assist steering gear. The output pressure demands on the pump can vary with the pump flow and system load restriction. This pressure must be limited to a predetermined maximum in order to protect the pump and other system components from damage. To this end, the pump contains a relief valve.
The relief valve includes a movable spring-biased spool that contains an internal poppet valve assembly. The spool normally covers a fluid pressure relief port. The spool on one side is exposed to the output pressure of the pump. Output pressure is also communicated through an orifice to the opposite side of the spool. The spool contains a venting passage in communication with the opposite side of the spool. The venting passage is normally blocked by the poppet valve assembly. When the output pressure of the pump exceeds a predetermined maximum amount, the poppet valve assembly opens to exhaust fluid through the vent passage to the pressure relief port. The reduced pressure on the opposite side of the spool allows the spool to move and open the pressure relief port, thereby relieving the output pressure.
The prior art poppet valve assembly includes a poppet spring that biases a poppet against a poppet seat. The force applied by the poppet spring determines the pressure at which the poppet valve assembly opens. The force applied by the poppet spring is determined by assembling the poppet valve assembly using shims to set the initial position of the poppet seat and, thus, the force applied by the poppet spring. Determining the correct number of shims to use in a particular poppet valve assembly is a time consuming and labor intensive process.